Page 352 - FUNDAMENTALS OF COMPUTER

NPP

352 Fundamentals of Computers NPP

Drawbacks of magnetic tape: Mwå~H$s` Q>on H$s hm{Z`m±…
1. This is a purely serial access memory. 1. `o nyU©V… loUr~Õ EŠgog _o_moar hmoVr h¡, Bg{bE
Therefore it is slower. Yr_r hmoVr h¡ &

2. Provisions must be made to keep the tape 2. Q>on S´>mBd H$s J{V H$mo pñWa aIZo hoVw AË`{YH$
drive speed constant. à~ÝY H$aZo hmoVo h¢Ÿ&

4.8 Optical Disks 4.8 Am°pßQ>H$b {S>ñH$
Optical disks are those in which LASER Bg_| boOa H$m Cn`moJ H$aHo$ OmZH$mar [aH$m°S>© H$s
beam is used to record as well as to read the OmVr h¡ d n‹T>r OmVr h¡Ÿ& BZH$s j_Vm ~hþV A{YH$ hmoVr
information. The storage capacity of an optical
disk is very high. It is cheaper and reliable. The h¡Ÿ& `o gñVr d {Q>H$mD$ hmoVr h¡ Ÿ& hoS> gVh Ho$ ñne© _|
read-write head is not in direct physical con- Z hmoZo go BZH$s Iam~ hmoZo H$s gå^mdZm H$_ hmoVr h¡
tact of the surface. Therefore chances of dam- Ÿ& bo{H$Z BZH$m EH$ hr AdJwU hmoVm h¡ {H$ BZH$m EŠgog
age are low. The single disadvantage of optical
disks is higher seek time. There are three types Q>mB©_ A{YH$ hmoVm h¡ Ÿ& VrZ àH$ma H$s {S>ñH$ Cn`moJ _|
of optical disks in use: AmVr h¢…
1. CD-ROM (Compact Disk Read Only 1. CD-ROM (H$m°ånoŠQ> {S>ñH$ arS> AmoZbr _o_moar)
Memory)
2. WORM (Write Once Read Many) 2. WORM (amBQ> dÝg arS> _oZr)
3. Erasable CD 3. BaoOo~b CD
CD - ROM CD- ROM
These optical disks are made up of poly- `o nmo{bH$m~m}ZoQ> go ~Zr hmoVr h¡& CÀM Vrd«Vm dmbo
carbonate. The high power LASER beam is boOa H$s ghm`Vm Xygar gVh na {N>Ð {H$E OmVo h¢Ÿ&
made incidence on this plastic (resin) surface to
burn a small hole (1µm). Intensity of light is boOa H$s Vrd«Vm 25 mw hmoVr h¡ Ÿ& Bg àH$ma go EH$
about 25mw for making tiny holes. The pre- _mñQ>a {S>ñH$ ~ZVr h¡ Ÿ& Bg _mñQ>a {S>ñH$ go EH$ _moëS>
pares a master disk. Now, a mold is prepared
from the master disk. Large number of plastic ~Zm {b`m OmVm h¡ Ÿ& Bg _moëS> H$s ghm`Vm go ~hþV
disk are stamped with the help of the mold. A gmar ßbmpñQ>H$ H$s JmobmH$ma gVhm| na N>moQ>o-N>moQ>o {N>Ð
thin layer of Aluminium is deposited because H$a {bE OmVo h¢ Ÿ& A~ BZHo$ D$na Eë`w{_{Z`_ H$s
this is reflective material. A transparent plastic
layer is deposited for protection. Thus, the in- naV M‹T>m Xr OmVr h¡ Ÿ& BgHo$ D$na ßbmpñQ>H$ H$s naV
formation in the CD-ROM is written in the M‹T>m Xr OmVr h¡ & AV… \o$ŠQ´>r _| hr Oê$ar OmZH$mar
factory. This pre recorded data can be read out CD-ROM _| ^a Xr OmVr h¡Ÿ& Bg OmZH$mar H$mo {g\©$
by the user. User cannot write anything. Thus,
it is Read Only Memory. n‹T>m Om gH$Vm h¡Ÿ&
Reading a CD-ROM CD-ROM H$mo n‹T>Zm
The small holes made by LASER beam are boOa go Omo N>moQ>o-N>moQ>o {N>Ð ~ZVo h¢, pits H$hbmVo
called pits and the unburned areas are called h¢ Ÿ& {OZ joÌm| na {nQ> Zht ~ZVo lands H$hbmVo h¢ &
lands. The whole surface is divided into pits 1 VWm 0 BÝhr {nQ>m| d b|S>m| Ho$ ê$n _| hmoVo h¢Ÿ& CD-
and lands representing 1 and 0. For reading the ROM H$mo n‹T>Zo Ho$ {bE BgHo$ D$na H$_ Vrd«Vm H$s boOa